Recently, there have been reported a lot of active researches on holographic digital data storage systems. Results of these researches are now being applied in such a field as a finger print recognition system, as well as any fields that take advantage of a large-scaled storage capability and a high-speed transfer rate.
A conventional holographic digital data storage (HDDS) system stores information on an interference pattern of an object beam and a reference beam in a storage medium, made of, e.g., photo-refractive crystal or photopolymer, which is sensitive to an amplitude of the interference pattern. The storage medium of the HDDS system is often fabricated in a disc shape, the so called “spinner disc” in the form of a CD or a DVD. The HDDS system stores information on an amplitude and a phase of the interference pattern beam by varying an angle of the reference beam. In this way, the spinner disc can store several hundreds or thousands of holograms, each of which is represented by a page in the form of a two dimensional array of binary digits.
When information stored in a storage medium are reproduced, wobbling may be resulted due to a misalignment between the storage medium and its axis of rotation, such that there exists an offset between the normal to the disc plane of the storage medium and the axis of rotation thereof.
Wobbling in certain storage media, e.g., conventional CD and DVD, can be corrected by using servo control of a head or a light pickup with a small mass. Since, however, the holograms are stored on a page-by-page basis in the spinner disc and the optics of the HDDS system is large, the wobbling compensation scheme of the conventional optical disc system cannot be used in the HDDS system.